(a) Technical Field
The present disclosure relates to an apparatus and method for manufacturing a metal separator for a fuel cell. More particularly, it relates to an apparatus and method for manufacturing a metal separator for a fuel cell, where the apparatus and method can manufacture large-sized metal separators in large quantities using, for example, metal plates such as stainless steel by thermoplastic deformation using an incremental and synchronized rubber molding process.
(b) Background Art
A polymer electrolyte membrane fuel cell (PEMFC) (also known as a proton exchange membrane fuel cell) has many advantages including a low operation temperature of about 80° C., and high energy efficiency, etc., when compared to other types of fuel cells.
Catalyst layers for promoting oxidation and reduction reactions may be coated on an oxidation electrode (anode) and a reduction electrode (cathode) of the PEMFC. At the oxidation electrode, supplied hydrogen is dissociated into hydrogen ions (protons) and electrons by the oxidation reaction. At the reduction electrode, the dissociated hydrogen ions combine with oxygen to suitably produce water and, at the same time, electric power is suitably generated by the oxidation reaction at the oxidation electrode and the reduction reaction at the reduction electrode.
A separator (also called a bipolar plate) may be used in the PEMFC and has a channel through which hydrogen and oxygen pass, and also functions to separate unit cells that are stacked in a fuel cell stack, and to support a membrane electrode assembly (MEA), and may also act as a current collector for collecting and transmitting generated electricity.
Preferably, the separator should have high electrical conductivity to minimize voltage loss, low gas permeability to prevent the supplied hydrogen and oxygen from permeating, low density, sufficient mechanical strength, excellent corrosion resistance in electrolyte used therein, good productivity, and low manufacturing cost.
FIG. 1 is a configuration diagram showing a fuel cell stack of a polymer electrolyte membrane fuel cell, in which a plurality of unit cell is stacked.
Each of the unit cells preferably comprises a polymer electrolyte membrane (also called a proton exchange membrane). An oxidation electrode (anode) and a reduction electrode (cathode) are suitably formed on both sides of the polymer electrolyte membrane. The polymer electrolyte membrane and the respective electrodes are integrally formed by a hot press, thus forming a membrane electrode assembly (MEA) 10.
Moreover, a gas diffusion layer (GDL) is suitably disposed on the outside of the electrodes of the MEA 10.
The MEAs 10 of adjacent unit cells are separated and supported by a separator 20.
FIG. 2 is a perspective view and a cross-sectional view showing a metal separator of FIG. 1. The metal separator (hereinafter referred to as a separator) 20 preferably has a metal plate structure that may further include fine grooves consisting of concave-convex parts 30 through which hydrogen as fuel and oxygen as oxidant suitably pass. The separator 20 is bonded to the GDL attached to the outside of the MEA.
The separator 20 functions to supply hydrogen to the fuel electrode, remove product water from the air electrode, and transmit generated electricity to an external circuit.
Accordingly, the structural factors such as the depth and width of the fine grooves of the separator may be considered to be important factors that have a considerable effect on the output efficiency of the fuel cell.
A stamping process has been widely studied as a process for manufacturing the separator for the PEMFC.
The stamping process is a simple process that enables mass production; however, in a process of molding a metal plate using an upper punch and a lower die, the shape accuracy of the separator depends on the dimensional accuracy of the punch and the die.
Moreover, during the stamping process of a fine metal pattern in which the upper punch moves down to deform the metal plate, a so-called recoiling phenomenon occurs, in which the flat part of the pattern is recoiled, since the upper and lower dies are in contact with the metal plate in a relatively narrow region of the separator.
After the upper punch has moved down, the deformation is concentrated in a relatively narrow region of the separator such as round parts of the top and bottom of the pattern, and thus excessive thinning occurs in the corresponding region. Furthermore, when the upper punch is removed upon the molding process of the metal plate, a so-called spring-back phenomenon occurs, in which part of the metal plate elastically recovers, and thus it is difficult to ensure the dimensional accuracy.
Further, forming an ultrafine pattern can be difficult, due to low accuracy of the pattern shape according to properties such as non-uniformity and anisotropy of the material and due to limitations in mold manufacturing technology and matching technology. Accordingly, the shape accuracy required by the fuel cell separator may not be satisfied. A new separator manufacturing method, which can minimize the above-described molding defects, is required.
Japanese Patent Publication No. 2006-114443 (the '443 reference herein), incorporated by reference in its entirety herein, is directed to a method for manufacturing a metal separator for a fuel cell using hydrostatic pressure.
The '443 reference teaches a method for manufacturing a metal separator through three examples to improve moldability of a metal plate material, the method including a process of forming a separator with one press stroke using urethane rubber for hydrostatic effect, a hydroforming process of applying hydrostatic pressure by applying liquid pressure to a confined space, and a process of applying hydrostatic pressure by applying liquid pressure to a liquid pressure pocket.
In the process of molding the separator, a metal material is placed on a lower die including a predetermined pattern used to mold the metal material and pressurized using urethane rubber or liquid pressure in a retainer provided at the top, thus forming the pattern on the metal material. As a result, a fine metal pattern can be formed by the above hydrostatic pressure process.
Since the thickness reduction rate of the metal material is uniformly distributed, it is possible to prevent the separator from tearing, which is caused by excessive local thinning which inevitably occurs in the stamping process. Further, since the urethane rubber is used instead of the upper punch made of metal, it is possible to process complex shapes without causing scratches on the surface of the metal plate, and it is also possible to mold a non-metal or coating plate.
The '443 reference is directed to a method for manufacturing a separator including a fine metal pattern formed by pressurizing urethane rubber, which is an elastic material, with one press stroke. However, in examples where a large-sized separator for the PEMFC is designed, a very high load is required, and the manufacturing speed is considerably reduced due to the pressure control for forming uniform fine patterns.
Further, sealing the retainer capable of controlling the liquid pressure by rubber or fluid may be difficult at very high molding pressure.
Further, the more the molding area is increased and the more the pattern shape is complicated with the use of the completely sealed retainer, the more the molding load for material filling is increased.
Accordingly, the process of using hydrostatic pressure to improve the moldability of the metal plate by applying low molding load is an important factor for improving productivity, and, further, the use of an open retainer instead of the completely sealed retainer can be used to optimize the process variables.
Japanese Patent Publication No. 2005-243252 (the '252 reference herein), incorporated by reference in its entirety herein, is directed to a method for manufacturing a metal separator, in which the moldability of the separator is improved by multi-stage molding in a stamping process using a press.
According to the '252 reference, a final separator including a pattern is not manufactured by a one-shot molding process. Instead, a preform is preferably formed by a primary molding process, and the thus formed preform is molded in a secondary molding process by a stamping process using upper and lower dies, thus manufacturing a final separator.
As taught by the '252 reference, the preform is suitably formed by the primary molding process in which an initial material is stamped by predesigned upper and lower molds, and the preform is used for the secondary molding process in which a final pattern is formed by bending and stretching the preform using the upper and lower dies. The use of such a preform may be more effective in improving the moldability of the material during the secondary molding process.
Further, with the use of the stretched preform, it is possible to prevent excessive local thinning, and thus the deformation rate is uniformly distributed.
The pattern shape of the separator taught in the '252 reference is complex, e.g. a serpentine structure, the molding depth is large, and it cannot avoid the above-described recoiling, local thinning, and spring-back, which are the fundamental problems of the stamping process used as a final manufacturing process so as to reduce the round parts and increase the flat part.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.